Switching and display elements containing ferroelectric liquid-crystal mixtures ("FLC light valves") are disclosed, for example, in EP-B 0 032 362. Liquid-crystal light valves are devices which modify their optical transmission properties, for example due to electrical switching, in such a way that light which is incident (and possible reflected again) is modulated in intensity. Examples are the known watch and calculator displays or liquid-crystal displays in the OA (office automation) or TV sectors. However, these also include light shutters, as employed in photocopiers, printers, welding goggles, polarizing spectacles for three-dimensional viewing, etc. So-called " spatial light modulators also fall within the area of application of liquid-crystal light valves (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054 and the papers cited therein).
Electro-optical switching and display elements (displays) are constructed in such a way that the FLC layer is surrounded on both sides by layers which are usually, in this sequence starting from the FLC layer, at least one alignment layer, electrodes and a limiting plate (for example made of glass). In addition, they contain one polarizer if they are operated in "guest-host" mode or in reflective mode, or two polarizers if the transmissive birefrigence mode is used. Switching and display elements may, if desired, contain further auxiliary layers, such as, for example, diffusion barrier layers or insulation layers.
Together with a distance between the limiting plates which is chosen to be sufficiently small, the alignment layers, which comprise an organic (for example polyimide, polyamide or polyvinyl alcohol) or inorganic (for example SiO) material, bring the FLC molecules into a configuration in which the molecules lie with their long axes parallel to one another and the smectic planes are arranged perpendicular or inclined to the alignment layer. In this arrangement, the molecules are known to have two equivalent alignments, between which they can be switched by applying an electrical field in a pulsed manner, i.e. FLC displays are capable of bistable switching. The response times are inversely proportional to the spontaneous polarization of the FLC mixture and are in the range of microseconds.
The major advantage of FLC displays of this type over the LC displays which are still usually encountered in industrial practice is regarded as being the multiplex ratio which can be achieved, i.e. the maximum number of lines which can be addressed in the time-sequential process ("multiplex process"), which is virtually unlimited in FLC displays, in contrast to conventional LC displays. This electrical addressing is essentially based on the pulse addressing mentioned above and described in illustrative terms in SID 85 DIGEST p. 131 (1985).
Particularly important functional parameters of an FLC display are
a) the maximum brightness (transmission in the bright state), PA1 b) the maximum contrast (ratio between the maximum transmission in the bright and dark states), PA1 c) the picture build-up rate (or the addressing rate of a pixel). PA1 .DELTA.n=difference between the refractive indices (uniaxial approximation) PA1 d=thickness of the FLC layer PA1 .lambda.=wavelength in vacuo PA1 .theta..sub.eff =effective tilt angle. PA1 case b: if n=2, the ring is in the main chain, PA1 case c: if n=3, the ring is in the polymeric network. ##STR2##
The object of the present invention is to provide alignment layers which, in ferroelectric liquid-crystal displays, result in improved brightness, lower residual transmission in the dark state and thus in a discrete improvement in the optical contrast.
In order to explain this object, the brightness (or transmission in the bright state), the transmission in the dark state, the contrast and the response time are described in more detail below.
The maximum transmission in the bright state T (bright) is, as is known, described for FLC displays to a good approximation by the equation (1): EQU T(bright)=sin.sup.2 (.pi..DELTA.nd/.lambda.) sin.sup.2 (4.theta..sub.eff)(1)
where:
In the ideal case, T(bright)=1 (=100%).
While the first of the two terms in equation (1) can be optimized relatively easily by matching .DELTA.n and d to the wavelength of visible light, the material-side optimization of sin.sup.2 (4.theta..sub.eff) causes problems since .theta..sub.eff is generally very much smaller than 22.5.degree. (optimum value).
In the so-called "chevron" geometry (see, for example, T. Rieker al., 11th Int. Liq. Cryst. Conf. Berkeley (1986)), in which the smectic layers are at an angle to one another, the currently available materials only have an angle of up to about .theta..sub.eff =8.degree., which results in a maximum transmission T(bright)=0.28 and thus corresponds to a loss of 72% of the light output of the FLC display illumination.
An exception with respect to the tilt angle is formed by FLC displays having an alignment layer of silicon monoxide (SiO) vapor-deposited at an angle, but this must be applied in a very complex and expensive vacuum process.
In the so-called "bookshelf" geometry (see, for example, H. R. Dubal al., Proc. 6th Intl. Symp on Electrets, Oxford, England (1988), and Y. Sato al., Jap. J. Appln. Phys. 28, 483 (1989)), in which the smectic layers are perpendicular to the glass plates, the special layer structure means that angles of approximately 22.5.degree. are achieved with the available liquid-crystalline materials. In this structure, a transmission of T(bright)=1 can be achieved.
As will be shown below, much greater effective tilt angles, and thus much greater transmission in the bright state, can be achieved using the alignment layers according to the invention in the "chevron" geometry than using conventional alignment layers.
The contrast is the ratio between the transmissions in the bright and dark switching states. At present, maximum contrast values of from 5 to 10 are given for FLC displays. The reason for these values, which are too low for many applications, for example TV, is both inadequate transmission in the bright state and excessive residual transmission in the dark switching state. The residual transmission can easily be detected between crossed polarizers from a bluish liquid-crystal structure. It is found for all FLC materials known hitherto if organic alignment layers, such as, for example, of rubbed polyimide or polyamide, are used. The causes are undesired non-uniformities of director (i.e. of the molecular preferential direction), which are known as twist states (see M. A. Handschy et al., Phys. Rev. Lett. 51, 471 (1983); M. Glogarova et al., J. Phys (France) 45, 143 (1984); N. Higi et al., Jap. J. Appln. Phys. 27, 8 (1988)). In the memory state and in multiplex mode, these non-uniformities result in a considerable reduction in the contrast of the display. In addition, the appearance of twist states is frequently associated with wavelength dispersion, which can result in distorted colors in the display.
It has already been attempted to suppress the appearance of the interfering twist states by a suitable choice of alignment layers, but hitherto only unsatisfactory results have been achieved. The virtually uniform states which sometimes appear (for example when silicon oxide vapor-deposited at an angle is used) very often proved to be unstable and relaxed again to give twist states.
The occurrence of twist states appears to be favored, in particular, if ferroelectric liquid-crystal mixtures of high spontaneous polarization are used (see M. A. Handschy et al., Ferroelectrics 59, 69 (1984)). However, such mixtures are particularly suitable for use in high-information displays since they result in short response times.
A further object of the present invention is to provide alignment layers which suppress the formation of twist states and thus enable the construction of ferroelectric displays of high brightness and high contrast.
The picture build-up rate or the picture change frequency is given by the number of lines of the FLC display and the duration of the electrical switching pulses. The shorter the pulses, the faster the picture build-up. On the other hand, the response time depends, on the material side, on the spontaneous polarization (P.sub.s) and the viscosity (.gamma.) of the FLC material.
Since the values for the rotational viscosity (.gamma.) can only be reduced to a very limited extent, an increase in (P.sub.s) is a suitable way of shortening the response time. However, this has hitherto failed due to reverse switching effects (probably caused by ionic impurities), which are described in the literature as "surface memory effects", "ghost pictures" and the like (cf., for example, J. DiJon al., SID conference, San Diego, 1988, pages 246-249 ).
The ionic impurities mean that a picture must be written a number of times so that the previous picture disappears completely ( "ghost picture"). However, this effect, which greatly impairs the usefulness, is more pronounced the higher the spontaneous polarization of the FLC material.
DE-A-3 939 697 and EP-A-0 385 688 have already presented FLC mixtures which, in order to avoid or reduce the "ghost picture effect" in displays, contain, as one component, a complex ligand for ions.
It has been found that, inter alia, numerous complex ligands are suitable for eliminating twist states. For example, DE-A-4 012 750 has proposed, for example, applying macrocyclic substances of various structure to conventional alignment layers in order to improve the contrast of a display, but this is associated with additional complicated production steps.